Purpose :
The calcium (Ca2+) feedback on phototransduction in rods and cones is critical for setting the sensitivity and kinetics of their photoresponses and extending their operating range to brighter background light. A major component of this feedback is the synthesis of cGMP by Guanylyl Cyclase (GC), which is regulated by Guanylate Cyclase Activating Proteins 1 and 2 (GCAP1/2). The distinct contributions of GCAP1 and GCAP2 to the physiology of mammalian cones have not been directly tested. To address this question, we examined the function of mouse cones lacking GCAP1, GCAP2, or both.

Methods :
We used ex vivo ERG recordings to examine the light responses of cones from isolated mouse retinas. The retinas were perfused with physiological Locke’s solution containing synaptic blockers and Barium to isolate the photoreceptor component of the ex vivo ERG signal. All mice were bred on Gnat1-/- background to block rod signaling and facilitate cone physiology recordings. The Ca2+-dependent regulation of cGMP synthesis and transduction CNG channel current was studied by exposing retinas to low ~30 nM [Ca2+].

Results :
Exposure of mouse retinas to low Ca2+ caused a transient >4-fold increase of the maximal cone response amplitude (Rmax) of WT cones. In contrast, mouse cones lacking both GCAP1 and GCAP2 did not show a large transient increase of Rmax in low Ca2+, indicating that this transient increase is mediated by GCAPs-dependent upregulation of cGMP synthesis. Notably, exposure of GCAP1-deficient cones to low Ca2+ produced a transient response increase comparable to that of WT cones indicating that GCAP2 can modulate cGMP synthesis efficiently in mouse cones. Consistent with this notion, steady-state light adaptation was also suppressed in cones lacking both GCAP1 and GCAP2, but normal in GCAP1-deficient cones. However, cone flash responses were slower than normal in the absence of GCAP1 suggesting that GCAP1 is required for the rapid dynamic modulation of cGMP synthesis in mammalian cones.

Conclusions :
GCAP2 can contribute significantly to the regulation of cGMP concentration and light adaptation in mammalian cones in the absence of GCAP1. Our biochemical modeling suggests that as little as 0.2 µM GCAP2 could explain the observed results in GCAP1-deficient cones, but it also indicates that in normal cones GCAP1-dependent regulation would largely exceed that of GCAP2.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.